I propose to investigate the mechanisms of mating type determination in the unicellular eukaryote Tetrahymena thermophila. After sexual reorganization (conjugation) cells start out with the genetic potential to express several mating types. Each cell, however, ultimately expresses only one of these. Tetrahymena, as a typical ciliate, maintains a separate germ line (the micronucleus) and soma (the macronucleus). Evidence obtained by Nanney and his collaborators over the past 25 years has led to the firm understanding that this mating type differentiation involves a regularly occurring hereditary differentiation of the macronucleus (exclusively) an event which shows chromosomal localization. This differentiation almost certainly involves a specific alteration of the somatic DNA. What needs to be done is a) to confirm directly the occurrence of the specific somatic DNA alterations, b) to determine the precise nature of the alteration (i.e., base modifications, base pair substitution or DNA rearrangement) and c) to unravel the biochemial mechanism involved in the alteration. We propose to conduct studies aimed at a) identifying, by mutation, genes in the micronucleus concerned with mating type determination (including structural genes for the mating type substances and genes specifying the altering system): b) determining (through mutation, microinjection or macronuclear relocation) the mating type gene composition of the macronucleus after differentiation, and c) probing the biochemistry of differentiation. The facility with which genetics and biochemistry can be carried out with Tetrahymena using bacteriological-like methods, the physical separability of the micro- and macronucleus on a preparative scale and the regular occurrence of the differentiating events in every conjugating pair at a definable stage combine to make T. thermophila a uniquely useful system for the study of developmentally controlled somatic DNA rearrangements.